Simple Experiments In Heat Reclamation: Part I – The Dishwasher

The original idea which started all these shenanigans (back all of two months ago, when heating oil was twice its current price) was that I would “downsize” my living quarters a bit for the winter, spending most of my time in the kitchen. Actually I spend most of my time in the kitchen anyway– it’s also sort of the living room– but the scheme was that I’d install some doors or insulating dividers so that the kitchen could be thermally closed off from the rest of the house, and heated independently when convenient.

One attraction of this is that, aside from the clothes dryer, most of the heat-emanating appliances are already in the kitchen: the stove, oven, dishwasher, microwave, fridge, and the only incandescent lights I can’t easily change to compact fluorescents. So it seemed, just in the course of living, that the kitchen was likely to stay pretty warm.

I hadn’t really anticipated that I still wouldn’t have turned the heat on at all by mid-December. But such is the case, because indeed the kitchen has stayed pretty warm (even though I haven’t finished installing the doors)(and by “pretty warm” I mean around 52F/11C). Just making coffee & oatmeal in the morning tends to heat the room about 3F. But as real cold weather approaches, I’ve turned thoughts to reclaiming heat that is currently being wasted. There are endless permutations of fancy, complicated, imaginative, labor-intensive and material-expensive ways to reclaim heat from various heat-wasting functions around the house. But my strong preference, at this stage, is for easy, non-permanent modifications which can be accomplished in under one hour for under $10 (or better, for free.) I set my sights, first, on the dishwasher.

Okay, I see everyone up in arms already: a dishwasher! Terrible waste of energy! You could just do your dishes by hand, saving gobs of energy! Yes, of course I could. And sometimes I do. I lived for many years without a dishwasher. But I enjoy having it. I only need to run it about twice a week. And once you finish reading this post, you might decide that it’s not evil– even if I just used it in the normal fashion.

In fact, I am first going to convince you that the dishwasher is environmentally friendly. Here are the facts on my dishwasher. When run on “light wash” cycle, without the “Hi-Temp Wash” or the SaniHeat™” the dishwasher runs for about 50 minutes. On these settings it uses no electrical heating– just the heat from the incoming hot water. It does have an electric motor and pump, which, according to the affixed placard, draw 1 amp (= 110 watts at 110V) when running. In the course of its hijinx, the dishwasher disgorges 5 gallons / 19L of water, at an average temp of 112F/44C (the first pre-rinse is cool, the remaining stages are hot.)

Perhaps those used to doing dishes by hand need a visual: here is 5 gallons of water, divided into two sides of my sink (like, you know, “wash” and “rinse” sides.)It’s certainly enough to do a couple days’ worth of dishes. But if you leave the water running while you wash, or if you rinse under running water, you’ll definitely use more than this much water. Also, to get the average temp of 112F, you could have nice hot water for the wash side (155F/68C) and cool water for rinse (69F/21C). Or you could have lukewarm water for both. You definitely can’t have hot for both.

Now, I could perhaps have washed all this in the sink-water above, but towards the end the wash-water would’ve been pretty icky, and the rinse-water pretty soapy. In order to match the water volume & heat efficiency of the washer, I would’ve had to refrain from “freshening up” either side. As far as electrical use– if I spent 15 minutes doing dishes, and had the kitchen lights on while doing so, then I would waste as much power as the dishwasher motor.

Not that the dishwasher motor power is wasted. It turns into heat. And heat is welcome here. The question (somewhere I started out on this question) is how not to lose that heat– plus, more significantly, all the heat from the hot water. Letting it go warm down the drain is a thermocrime. In sink-washing, you could reclaim the heat by just letting the dirty water sit in the sinks overnight, or however long it takes to reach room temp. But many people might find this unsavory– plus, your sink is out of commission for hours. A better approach might be to use dishpans, and set them aside on the counter after to cool. Not a bad idea.

But with the dishwasher, it turned out to be surprisingly easy to reclaim the water heat. I found the machine has a flexible plastic drain hose, running under the sink, then stuck in a PVC drain pipe. It was easy to pull it out. Then, just as easy to put it into an old bucket instead. Ta da! 5 gallons of warm water, nicely contained. I tossed a plastic lid on it (to prevent steaming evaporation), moved it out of the way, and let it cool. When I remembered, I poured it down the drain.

How much energy can you save this way? Well, that depends how cold you keep your house. The colder your house, the more heat you get back. (Do you detect proselytizing?) The best case is if your kitchen, overnight, gets colder than your incoming tap water– which I’m pleased to report mine does these days. In that scenario, you actually get a bit more heat back than you put in! If my kitchen drops to 45F/7C, then I reclaim (44C – 7C =) 37˚C worth of heat, from all 19L of water. That works out to 703 kcal, or 0.82 kwh. There’s a bit more, too: some of the heat from the original hot water went into heating the dishes, and the dishwasher racks and walls, all of which ultimately gets delivered to the kitchen, too. (Knowing that my hot tap water is 125F/52C, but comes out the dishwasher at 44C, can quickly calculate that the objects absorbed and will return another 152 kcal). All told, this reclamation represents, roughly, 2/3 cup of oil burned in the furnace– or running the space heater on “MEDIUM for an hour– per run. Even at my paltry rate of dish use, that comes to over two gallons of oil a winter, or 50 hours of space heater. And, more importantly, it might stave off, for a few more days, turning the heat on in the first place.

There are many ways to improve the convenience (though not the efficiency) of this “system”. I could, for example, find a metal bucket– that would speed the wastewater cooling, so it wouldn’t have to be sitting around as long. I could find a 5-gallon container that fits under the sink. I could “hard-wire” it into the drains, so it could be emptied with a foot-pump and never seen by houseguests. But that’s all so complicated. I like the bucket.

[P.S. There is one wrinkle I forgot to mention: no matter how you do dishes, drying them will cost you some heat. And, alas, it’s the same amount of energy no matter how you choose to do it.]

[P.P.S. It has been brought to my attention that standard dishwasher soap may be more environmentally unfriendly than dish hand-washing soap. I am investigating “greener” soap options…]

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5 Responses to “Simple Experiments In Heat Reclamation: Part I – The Dishwasher”

Interesting. Your dishwasher is obviously better than mine, since mine would not “wash” dishes if I did not rinse them thoroughly before I put them in (read “wash by hand”). Thus I simply omit the second step, the part with the dishwasher. But if mine could handle what yours can, I would be using it too. And still handwashing my favourite coffee cup, I need it more than twice a week. What I particularly like about this post is that it seems to the first steps beyond “cold house = less oil in furnace = money saved”, at least in terms of what comes through on this end. And I like. Perhaps, once the cold season is done, you could continue it as green house journal. So many more places to fiddle and tweak.

“What I particularly like about this post is that it seems to the first steps beyond “cold house = less oil in furnace = money saved”Johanna, have you not heard me extolling on the other virtues of the cold house? The quietness? The improved humidity? The reduction in drafts? The good sleeping? The extra-friendliness of the cat?

The improved humidity was then linked to not feeling as cold again, ditto on reduction in drafts. I’m not sure how “quiet” or a shivering cat that has no choice but to sit on your head for the extra BTU output and thus lead you to be less cold while sleeping and consequently sleeping better are green house things, though they are certainly a nice benefit…

What!!! You poured 5 useful gallons of room temp water down the drain!! Use it to flush your toilet–just dump it into the bowl and it will flush. It will save heating your tank water to room temp and save water.

[…] body, not in the walls and or the “systems room”– and the rest is rather cheap (cast-off joint compound buckets, for example). I’d like to appeal to people who would like to save the world (or at least, lower their […]